1. Field of the Invention
The present invention relates to a diaphragm valve to be used for opening and closing a flow path of a fluid piping, such as various liquid piping in food industry, pharmaceutical industry and so forth.
2. Description of the Related Art
FIG. 8 shows the conventional diaphragm valve. The diaphragm valve is constructed by fixing a peripheral edge portion on a side of a diaphragm 4 arranged in opposition to a valve seat 3 which is arranged within a valve body 1, on an annular diaphragm mounting seat 2 provided on a side wall portion la of the valve body, for opening and closing a flow path by moving the diaphragm 4 toward and away from the valve seat 3 by driving in forward and reverse direction an actuation shaft of a valve operating portion 5 connected to a central position on a back surface side of the diaphragm 4. The valve operating portion 5 is formed with an air cylinder. A piston rod of the air cylinder serves as the actuation shaft. The air cylinder is mounted on the valve body 1 via a bracket 8.
As shown in FIG. 8, in the conventional diaphragm valve, the peripheral edge portion of the diaphragm 4 is fixed to the diaphragm mounting seat 2 by clamping the peripheral edge portion of the diaphragm between the diaphragm mounting seat 2 and a bracket mounting flange 6 of the valve operating portion 5 and by tightening four bolts 7 through the flange 6, the peripheral portion of the diaphragm 4 and the mounting seat 2. If tightening force of the bolts 7 is small, the force compressing the peripheral portion of the diaphragm onto the mounting seat is insufficient to prevent leakage of liquid. If the tightening force of the bolts 7 is too large, the force compressing the peripheral edge of the diaphragm locally becomes large in the portion around the bolts 7, forming gaps between the diaphragm 4 and the mounting seat 2, between adjacent bolts 7, to cause leakage of the liquid. Therefore, adjustment of tightening force is difficult. Furthermore, there is sufficient personal error in the tightening operation to cause significant difficulty in properly fixing the peripheral portion of the diaphragm 4 onto the diaphragm mounting seat 2.
On the other hand, as shown in FIG. 9, the diaphragm 4 is formed with a surface membrane 4a of about 1 mm thick made of fluorine containing resin having superior chemical resistance, high water resistance and surface slip characteristics, such as Teflon (polytetrafluoroethylene: Registered Trademark of Du Pont). The back surface of the membrane 4b is made of rubber and supports the surface membrane 4a. The rubber back surface membrane 4b causes little thermal expansion and contraction in response to temperature variation. On the other hand, the surface membrane 4a of fluorine containing resin, such as Teflon, has a property to expand up to a certain temperature and to cause shrinking at higher temperature. Therefore, sufficient shrinking margin W is provided for a peripheral edge portion of the surface membrane 4a to be mounted on the mounting seat 2 of the valve body 1.
As shown in FIG. 10, an annular ridge 9 is formed along the peripheral edge portion of the surface membrane 4a for enhancing the sealing effect with the diaphragm mounting seat 2 and the valve seat 3. Also, a linear ridge 10 extends diametrically in relation to the annular ridge 9 for the valve seat. The distance between the annular ridge 9 and an inner end edge of the diaphragm mounting seat 2 is the shrinking margin W. Thus, as can be appreciated from FIG. 9, if the shrinking margin W is wide, a gap S formed between the liquid contacting surface of the membrane 4a and the diaphragm mounting seat 2 becomes deep, increasing tendency of penetration and trapping of the liquid therein to cause difficulty of washing. When the liquid is a food, such as milk, it becomes quite insanitary. It should be noted that, in FIG. 10, the reference numerals 4c designate bolt insertion holes provided at four corners of the diaphragm 4.
The present invention has been worked out in view of the problems set forth above. It is an object of the present invention to provide a diaphragm valve which can properly mount the peripheral portion of a diaphragm over the entire periphery and establish with certainty a seal between the peripheral portion of the diaphragm and a diaphragm mounting seat, for effectively preventing leakage of a liquid.
Pursuant to one aspect of the invention, a diaphragm type liquid control valve is provided with an improved means for securing and sealing the diaphragm. This includes an annular diaphragm mounting seat on the main valve body, in combination with an annular projection extending from a bracket mounted on the opposite side of the diaphragm from the mounting seat. The bracket and valve body are secured together and positioned by mating surfaces on the valve body and the bracket which position the annular projection a known distance from the diaphragm mounting seat. Proper positioning of the bracket is assured by the before mentioned mating surfaces, to reliably secure and seal the diaphragm in the valve structure. The diaphragm is thus not penetrated by bolts, providing a superior seal which is not affected by personal error of the valve assembly personnel. Additionally, the diaphragm, which may include costly materials, may be of circular form, requiring less material in its production.
According to another aspect of the invention, the diaphragm element, which may have a liquid-side membrane formed of a fluorine containing resin (e.g., Teflon) is formed with an annular peripheral projection received within an annular recess surrounding the diaphragm mounting seat. This arrangement significantly limits the shrinkage effect on the fluorine containing membrane, when the diaphragm is exposed to heated liquid. Additionally, the liquid side of the membrane may be provided with an annular ridge, positioned to engage the diaphragm mounting seat to provide reliable sealing contact. The arrangement of the annular projection on the diaphragm and the receiving recess surrounding the diaphragm mounting seat enables the annular projection to be positioned more closely than otherwise to the inner edge of the diaphragm seat, minimizing the space for stale liquid to collect in that region and facilitating the clean out of the valve when necessary. This is particularly advantageous for valves used in food processing systems or the like, where sanitation is an important factor.
According to a further aspect of the invention, the valve structure, on the back side of the diaphragm, is provided with a leak-sensing element, to quickly detect any failure in the diaphragm. The sensor includes a generally sealed sensor casing exposed to the inside of the valve, behind the diaphragm, and provided with an exposed infiltration membrane, which permits liquid to infiltrate from outside to inside of the casing, but preferably not in the other direction. Within the casing, there is a strong electrolyte associated with a pair of spaced-apart electrodes. When the electrolyte is contacted by liquid infiltrating the membrane, a current can flow between the electrodes to operate an alarm. This provides for reliable leak detection, even when the valve is functioning with relatively nonconductive liquids, such as highly pure water.
Preferably, the diaphragm may be comprised of a liquid-contacting membrane of fluorine containing resin and a support membrane of rubber laminated on back surface of the liquid-contacting membrane. The support membrane is provided with leakage indicating conduit which enables leaking liquid to quickly come into contact with the sensor element.
Other objects and advantages of the invention will become more clear from the discussion given hereinafter.
The present invention will be understood more fully from the detailed description given hereinafter with reference to the accompanying drawings of preferred embodiments of the present invention, which, however, should not be taken to be limitative to the present invention, but are for explanation and understanding only.